This invention relates to an outboard motor cooling arrangement and more particularly to an improved cooling system for the lower unit of a marine propulsion device.
As is well known, many forms of outboard marine propulsion devices exhaust the burnt gases from the powering internal combustion engine to the atmosphere through an underwater high-speed exhaust gas discharge. These discharges are positioned in the lower unit and frequently discharge the exhaust gases through the hub of a propeller which forms the propulsion device for the outboard unit. By utilizing such underwater exhaust gas discharge systems it is possible to provide cooling and silencing of the exhaust gases.
Conventionally, the lower unit is formed with an outer casing. There is provided at the lower portion of this outer casing an exhaust gas chamber to which the exhaust gases are delivered through an exhaust system. It is from this chamber that the exhaust gases exit through the underwater exhaust gas discharge. Frequently, the exhaust system that delivers the exhaust gases to this chamber is encircled at least partially by a cooling jacket through which water which has passed through the cooling system of the engine is delivered. In this way excessive heat transfer to the lower unit outer casing is avoided.
However, at least a portion of the exhaust cavity formed in the lower unit is in direct contact with a portion of the outer casing and is not otherwise cooled. This portion of the outer casing is frequently positioned above the water level under normal high-speed operation. However, water does come into contact with this area during operation of the watercraft, as should be obvious.
Thus, a condition exists where there is a highly heated portion of the lower casing in close proximity to the water level. When the water contacts this heated portion, it will be vaporized, and thus leaves deposits on the lower unit casing. When operating in marine environments, this deposit is primarily salt, and thus provides not only problems with corrosion, but results in an unsightly appearance.
It is, therefore, a principal object of this invention to provide an improved arrangement for the exhaust system of such lower units.
It is another object of this invention to provide an arrangement wherein the outer casing of the lower unit is maintained at a low enough temperature that deposits cannot be formed thereon.
It is a further object of this invention to provide an improved arrangement for cooling the lower unit outer casing of a marine propulsion unit through which the exhaust gases pass.